Magnetic separators are widely used to separate magnetic from non-magnetic materials. Magnetic separating systems have been used for some time to separate "particles" as large as crushed ore and to separate very fine, powder-like particles from a mixture containing both a primary product and the magnetic particles. Grate magnets are only one of a large class of magnetic separators and, typically, include several parallel V-shaped slots, each such slot opening to a magnet positioned just below and parallel to it.
Conventional grate magnets are attended by a number of problems. A major problem is that the magnetic separators must be cleaned periodically so that their separating capability (as represented by the "attracting power" of the magnetic field) is retained. And, of course, downtime needed for cleaning interrupts the smooth flow of the separating "line" and increases production costs.
If an electromagnet provides the separating magnetic field, such magnet can simply be turned off and the magnetic particles fall away. Residual magnetism can remain, however, and very small particles may not fall from the separator. And, presumably, the electromagnet is de-energized only under circumstances where the falling particles cannot contaminate the material from which they have been separated.
Another approach for cleaning grate magnets involves a magnetic rod positioned concentrically within a non-magnetic outer tube. For cleaning, the magnetic rod is withdrawn to permit magnetic particles to fall away. This principle is described in U.S. Pat. No. 4,867,869(Barrett).
The arrangement shown in U.S. Pat. No. 4,784,759(Elliott) uses a rotating, screw-like conveyor positioned inside a hollow stationary tube. A magnetic field is provided along the tube by four bar-like magnets which extend the length of the tube and are positioned around the outside of the tube about 90.degree. from one another. The resulting force field is substantially symmetrical about at least eight planes, any of which includes the axis of rotation. Magnetic material is attracted to the inner side of the tube and urged upward for discharge. The screw conveyor is reversible and material can also be fed downward. In another embodiment, only two bar magnets are used along the length of the tube.
The arrangement shown in U.S. Pat. No. 4,818,378(Elliott) is something of an "inside out" version of that shown in the Elliott '759 patent discussed above. That is, a single rotating rod-like bar magnet is concentric within a stationary tube wrapped with one or two spiral ramps. As the interior magnet rotates, the magnetic material is advanced along the outer surface of the stationary outer tube.
The apparatus shown in U.S. Pat. No. 3,595,391(Schmid) uses an inclined rotating drum, the interior of which is fitted with spiral ribs. The mixture bearing the magnetic particles is placed inside the drum for separation. A magnet is positioned outside the drum and the attracted magnetic particles adhere to the side of the drum, are carried upward away from the magnet and finally dropped to the bottom of the drum. This progressive lifting and dropping of magnetic material causes the magnetic material to move toward the lower end of the drum as the non-magnetic material moves towards the upper end and is discharged.
The apparatus shown in U.S. Pat. No. 1,340,457(Newton) is very similar in operating principle to that shown in the Schmid patent. The common operating principle involves the use of a localized magnetic field provided by an electromagnet at one end of the separator and a rotating drum with spiral ribs to urge material in a particular direction.
A problem with magnetic separators of the foregoing types is that they must be taken out of service to be cleaned. In other words, production halts during cleaning. Another problem with certain known magnetic separators is that they tend to be suitable for only a relatively narrow range of magnetic particle sizes. If a magnetic separator is configured to attract and hold large particles, it is correspondingly less likely to give up its magnetic "fines" when cleaned.